Assertion: Ethanol is a weaker acid than phenol. Reason: Sodium ethoxide may be prepared by the reaction of et

NCERT Exemplar · Q72

Q. Assertion: Ethanol is a weaker acid than phenol. Reason: Sodium ethoxide may be prepared by the reaction of ethanol with aqueous NaOH.

(i) Assertion and reason both are correct statements and reason is the correct explanation for assertion.

(ii) Assertion and reason both are correct statements but reason is not the correct explanation for assertion.

(iii) Assertion is correct statement but reason is wrong statement.

(iv) Assertion is wrong statement but reason is correct statement.

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Concept understanding — Acidity Of Phenol

Inductive Effect on Acidity – From Intuition to Precision

Imagine you are holding a rope tied to a heavy box. If you pull the rope, the box moves toward you. Now imagine the rope is made of rubber bands — the pull still reaches the box, but it gets weaker the farther away you are. That is exactly how the inductive effect works inside a molecule.

The Core Intuition

An acid donates a proton ( $H^+$ ). After it does, the remaining part (the conjugate base) carries a negative charge. The stability of that negative charge determines how willing the molecule is to give up the proton. More stable conjugate base → stronger acid.

Now, some atoms or groups are electron-withdrawing — they pull electron density toward themselves through the sigma bonds. If such a group is attached near the acidic proton, it pulls some electron density away from the negative charge on the conjugate base. That spreads out (delocalises) the negative charge, making the conjugate base more stable. The acid becomes stronger.

Conversely, electron-donating groups push electron density toward the negative charge, concentrating it and making the conjugate base less stable. The acid becomes weaker.

Note

The inductive effect operates through sigma bonds only. It does not involve pi bonds or resonance. It is a permanent, through-bond polarisation.

The Precise Statement

Inductive effect on acidity: The acidity of a compound increases with the presence of electron-withdrawing groups (EWGs) near the acidic site, and decreases with electron-donating groups (EDGs). The effect is strongest when the group is closest to the acidic proton, and diminishes rapidly with distance.

Mathematically, for a series of substituted carboxylic acids:

\text{R-COOH} \quad \text{where R = substituent}

The acid dissociation constant $K_a$ changes as:

If R is electron-withdrawing (e.g., $-\text{Cl}$ , $-\text{NO}_2$ , $-\text{CF}_3$ ): $K_a$ increases → stronger acid.
If R is electron-donating (e.g., $-\text{CH}_3$ , $-\text{C}_2\text{H}_5$ ): $K_a$ decreases → weaker acid.

Why Distance Matters

The inductive effect falls off with distance because sigma bonds are localised. Each bond attenuates the effect by roughly a factor of 2–3. For example, compare:

Compound	$pK_a$	Explanation
$\text{CH}_3\text{COOH}$	4.76	Reference (no EWG)
$\text{ClCH}_2\text{COOH}$	2.86	Cl withdraws through one bond
$\text{Cl}_2\text{CHCOOH}$	1.29	Two Cl atoms, stronger withdrawal
$\text{Cl}_3\text{CCOOH}$	0.65	Three Cl atoms, strongest withdrawal
$\text{CH}_3\text{CH}_2\text{COOH}$	4.87	Ethyl group is electron-donating (slightly weaker acid)

Notice: $\text{ClCH}_2\text{COOH}$ is about 100 times stronger than acetic acid ( $\Delta pK_a \approx 1.9$ ). But if the Cl is moved further away:

Compound	$pK_a$
$\text{ClCH}_2\text{CH}_2\text{COOH}$	4.08
$\text{ClCH}_2\text{CH}_2\text{CH}_2\text{COOH}$	4.52

The effect fades as the chlorine moves farther from the carboxyl group. …

Why this formula?

Acidity of Phenol: Why It's More Acidic Than Alcohols

Let's build this from first principles — understanding why phenol is acidic is the key to mastering organic chemistry.

1. The Core Observation

Phenol ( $\ce{C6H5OH}$ ) has a pKa ≈ 10, while ethanol ( $\ce{CH3CH2OH}$ ) has a pKa ≈ 16.

This means phenol is about 1 million times more acidic than a typical alcohol.

The question: Why does the O–H bond in phenol break so much more easily?

2. The Key: Stability of the Conjugate Base

Acidity is determined by the stability of the conjugate base after losing $\ce{H+}$ .

Alcohol conjugate base: $\ce{CH3CH2O-}$ (alkoxide ion) — negative charge is localized on oxygen.
Phenol conjugate base: $\ce{C6H5O-}$ (phenoxide ion) — negative charge is delocalized into the benzene ring.

The Resonance Explanation

The phenoxide ion has multiple resonance structures:

\ce{C6H5O-} \leftrightarrow \text{(several resonance forms where negative charge moves to ortho/para carbons)}

Draw the structures mentally:

One structure has the negative charge on oxygen.
Other structures show the negative charge on carbon atoms at the ortho and para positions of the ring.

This delocalization spreads the negative charge over more atoms, making the ion more stable.

Key principle: The more stable the conjugate base, the stronger the acid.

3. Why Alcohols Can't Do This

In an alkoxide ion ( $\ce{RO-}$ ), the negative charge is stuck on oxygen.

There are no empty p-orbitals or conjugated $\pi$ systems nearby to accept the charge.

Result: The alkoxide is less stable, so the alcohol is less acidic.

4. The Inductive Effect Also Helps (But Resonance Dominates)

The benzene ring is slightly electron-withdrawing (due to its $sp^2$ carbons being more electronegative than $sp^3$ ).

This inductive effect pulls electron density away from the O–H bond, making the proton slightly more positive and easier to remove.

However, resonance stabilization of the conjugate base is the dominant factor — inductive effects alone cannot explain the million-fold difference.

5. The Quantitative Picture (pKa Values)

Compound	pKa	Conjugate base stability
Ethanol	~16	Localized charge on O
Phenol	~10	Delocalized charge via resonance
Acetic acid	~4.76	Even more resonance (two O atoms)

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The assertion -- ethanol is a weaker acid than phenol -- is correct, because the phenoxide ion is resonance-stabilised while the ethoxide ion is not. But the reason -- that sodium ethoxide can be prepared by reacting ethanol with aqueous NaOH -- is factually wrong: aqueous NaOH cannot effectively deprotonate ethanol, because water is itself a slightly stronger acid than ethanol. The correct option is (iii).

Why This Question Tests Conceptual Depth

This assertion-reason question checks two separate things: whether you know ethanol is a weaker acid than phenol, and whether you know how sodium ethoxide is actually prepared.

1. The assertion: ethanol vs. phenol acidity

Acidity depends on the stability of the conjugate base after losing H+.

Phenol ( $C_6H_5OH$ ) loses H+ to form the phenoxide ion ( $C_6H_5O^-$ ), whose negative charge is delocalised into the aromatic ring by resonance. This stabilisation makes phenol give up its proton comparatively easily.
Ethanol ( $CH_3CH_2OH$ ) loses H+ to form the ethoxide ion ( $CH_3CH_2O^-$ ), where the charge stays localised on oxygen -- no resonance is possible.

Relative acidity: Phenol (pKa ~10) is stronger than Ethanol (pKa ~16). The assertion is correct .

2. The reason: can sodium ethoxide be made from ethanol + aqueous NaOH?

This part is wrong. Consider the equilibrium:

CH3CH2OH + NaOH <=> CH3CH2ONa + H2O

For this to proceed usefully to the right, hydroxide would have to be a better base than ethoxide -- i.e. water would have to be a weaker acid than ethanol. The opposite is true: water's pKa (~15.7) is slightly lower than ethanol's (~16), meaning water is the marginally stronger acid and hydroxide the weaker base. So the equilibrium lies almost entirely on the reactant side -- aqueous NaOH does not give a useful yield of sodium ethoxide. …

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Method: Conceptual Evaluation of Assertion-Reason Questions

This method tests your understanding of why a statement is true, not just whether it is true.

Steps:

Evaluate the Assertion independently
- Assertion: Ethanol is a weaker acid than phenol.
- Correct. Phenol ( $\text{C}_6\text{H}_5\text{OH}$ ) is more acidic than ethanol ( $\text{C}_2\text{H}_5\text{OH}$ ) because the phenoxide ion ( $\text{C}_6\text{H}_5\text{O}^-$ ) is stabilised by resonance with the benzene ring, while the ethoxide ion ( $\text{C}_2\text{H}_5\text{O}^-$ ) has no such stabilisation.
- ✓ Assertion is true.
Evaluate the Reason independently
- Reason: Sodium ethoxide may be prepared by the reaction of ethanol with aqueous NaOH.
- Correct. Ethanol reacts with aqueous NaOH to form sodium ethoxide and water:

\text{C}_2\text{H}_5\text{OH} + \text{NaOH} \rightarrow \text{C}_2\text{H}_5\text{ONa} + \text{H}_2\text{O}

 This is an acid-base reaction where ethanol (weak acid) donates a proton to the hydroxide ion.

✓ Reason is true.

Check if the Reason explains the Assertion …

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Q.Assertion: Ethanol is a weaker acid than phenol.
Reason: Sodium ethoxide may be prepared by the reaction of ethanol with aqueous NaOH.

Concept understanding — Acidity Of Phenol

Inductive Effect on Acidity – From Intuition to Precision

The Core Intuition

The Precise Statement

Why Distance Matters

Why this formula?

Acidity of Phenol: Why It's More Acidic Than Alcohols

1. The Core Observation

2. The Key: Stability of the Conjugate Base

The Resonance Explanation

3. Why Alcohols Can't Do This

4. The Inductive Effect Also Helps (But Resonance Dominates)

5. The Quantitative Picture (pKa Values)

Why This Question Tests Conceptual Depth

1. The assertion: ethanol vs. phenol acidity

2. The reason: can sodium ethoxide be made from ethanol + aqueous NaOH?

Method: Conceptual Evaluation of Assertion-Reason Questions

Steps:

Common Mistakes & How to Avoid Them (Acidity of Phenol vs Ethanol)

Mistake 1: Confusing the Reason's Validity

Mistake 2: Thinking the Reason Explains the Assertion

Mistake 3: Forgetting the Actual Explanation for Acidity Difference

Mistake 4: Misreading the Options

Q.Assertion: Ethanol is a weaker acid than phenol. Reason: Sodium ethoxide may be prepared by the reaction of ethanol with aqueous NaOH.

Why This Question Tests Conceptual Depth

1. The assertion: ethanol vs. phenol acidity

2. The reason: can sodium ethoxide be made from ethanol + aqueous NaOH?

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Q.Assertion: Ethanol is a weaker acid than phenol.
Reason: Sodium ethoxide may be prepared by the reaction of ethanol with aqueous NaOH.

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